电气工程及其自动化专业英语（2）
电气工程及其自动化专业英语 Specialized English for Electrical Engineering Its Automation 戴文进 主 编 杨植新 副主编,Part 2 Electric Machinery,Unit 11 The Transformer on No Load Unit 13 Construction and Application of D.C. Machines Unit 16 Three-Phase Induction (Asynchronous) Machine with the Rotor at Standstill· Unit 19 Armature Reaction of Salient-Pole Synchronous Machine a Two-Reaction Theory,Unit 11 The Transformer on No Load-Specialized English Words,in close proximity (to) 紧密耦合 permeability 磁导率 magnetizing current 激磁电流 power component of current 电流的有功分量 sinusoidal variations 正弦变量 a mutually induced e.m.f. 互感电动势the applied voltage 外施电压zero-power-factor 零功率因数the no-load power factor 空载功率因数formulate 用公式表示,系统地阐述saturation 饱和,Unit 11 The Transformer on No Load,The Simple Two-Coil Transformer The transformer is a straightforward application of Faraday's Law of Electromagnetic Induction. The simple transformer consists of two coils in close proximity. One coil of N1 turns is excited with alternating current and therefore establishes a flux 11 which alternates with the current (随时间交变）. The other coil is linked by （与-交链）most of this flux and thus has a mutually induced e.m.f. of value e2=N2d21/dt. This e.m.f. would drive a load current through any circuit connected to the terminals of the second coil. Energy would then be transferred through the medium of the magnetic field（磁场媒介） from coil 1 to coil 2.,The transformation could be from any convenient input voltage to any convenient output voltage. This apparently simple function of the transformer makes it as vital to modern industry as the gear train which, as a “transformer” of speed and torque represents an interesting analogy（句子的主语是“function”，谓语为“make”，“as vitalas”意为“与一样重要”，“which”引导一个定语从句“which represents an interesting analogy as atransformerof speed and torque”，修饰“gear train”，意为“齿轮箱与变压器表现出一种有趣的相似，齿轮箱被称为速度和转矩的变换器。” The coils would be on an iron core (铁芯）and so at the cost of introducing iron losses, the value of flux per ampere is increased several hundred times because of the change of permeability from 0 to 0 r .,The exciting or magnetizing current （励磁电流）can thus be very small. Further, the proportion of the total flux which is linked mutually by the two coils is greatly increased. Looked at another way, the leakage flux 1 which links N1 turns alone and “leaks” between the two windings without linking the N2 turns, is a much smaller fraction of the total, making for(有助于,倾向于) more efficient energy transfer.The mutual flux would be a much larger percentage of the total and the flux pattern (磁力线图)would be rather more involved(复杂的), but simplified diagrams of this kind are quite adequate for the understanding of the principles to be discussed in this chapter.The exciting coil which initiates (激励)the flux changes is called the primary winding, and the coupled coil which receives energy as a result (因此), is called the secondary winding.,It should not be difficult to realize that the two functions are interchangeable: if coil 2 were excited instead, a mutual e.m.f. would be induced in coil 1 which would then become the secondary winding(二次绕组). It should be obvious that since the primary winding also experiences flux changes, there is a primary e.m.f. self-induced this time. With winding 1 excited, the primary e.m.f. would be equal to N1d 11/dt. This is practically in phase opposition to (与-反相)the applied terminal voltage V1 and limits the current to a very much smaller value than V1/R1, where R1 is the primary resistance(原边电阻). As explained later, the resistance drop is normally very small so that the back e.m.f. is virtually equal in magnitude to V1.,If the applied voltage is alternating sinusoidally at frequency f=/2 Hz, a vector diagram can be drawn, and for an air-core transformer, neglecting resistance. As shown I1 is the magnetizing current producing 11 which rises and falls with it in time phase. It is therefore in quadrature with (与-正交)V1 and the circuit is a zero-power-factor lagging load. With the secondary circuit open, the primary coil is an inductor whose voltage drop is +N1d 11/dt or in vector form=j L11I1.The two components of flux linking N1 (与N1交链的磁通)are shown and though I1 is in phase with both of them,it will be found that when the secondary is carrying current, the mutual flux is no longer in phase with the primary current due to the reaction of the secondary ampere-turns(磁势).,The phase of component 1 is unchanged, however, since it is produced by primary current alone and is therefore in phase with it. Further, even with an iron core the reluctance offered to 1 is still predominantly due to its path in air so that the leakage flux remains virtually proportional to the primary current providing(=provided) the iron path is not greatly saturated(饱和). The e.m.f. N1d 1/dt may thus be treated as if it were due to an unsaturated inductance of magnitude N121. The corresponding leakage reactance is a dominant factor in transformer behavior(characteristic).For the iron-cored transformer which will form the subject of this chapter, the total back e.m.f. of the primary, N1d 11/dt, will be considered in two components, the one due to leakage and the other due to the mutual flux(互感磁通).,
